A Global Navigation Satellite System (GNSS) receiver requires information from at least four GNSS satellites in order to determine its three dimensional position. The approximate distance between a GNSS satellite and a receiver is generally referred to as a pseudorange. The location of the GNSS satellites relative to the receiver has a major impact on the overall accuracy of the determined three dimensional position. In general, good satellite geometry occurs when satellites are evenly scattered relative to the receiver. When less than four GNSS satellites are acquired or in a case of bad satellite geometry, a stand-alone GNSS receiver may fail to fix its three-dimensional position. Difficulty in acquiring satellites is typically due to signal degradation and unavailability of satellites in challenging environments such as indoors, in densely forested areas or in deep urban locations, for example, where attenuation and multipath effects make it difficult for the GNSS receiver to discriminate and acquire information from the required number of GNSS satellites.
Other non-GNSS positioning techniques use several technologies such as WiFi and Cellular-based positioning, for example, which can provide a source of additional information to assist a stand-alone GNSS receiver when it cannot acquire a sufficient number of GNSS satellites or in the case of bad satellite geometry. Cooperation between a GNSS receiver and any other positioning technology is referred to as Assisted-GNSS (A-GNSS). The assistance information generally includes ephemeris data (real or synthetic), timing data and/or position estimation.
A well known GNSS is the Global Positioning System (GPS). In conventional Assisted-GPS (A-GPS) where initial position assistance is available, the initial position assistance may be used as an aid by the receiver to allow the receiver to focus on signals from satellites passing overhead. More generally, however, the initial position assistance just provides a fallback position in case the GPS receiver still fails to acquire information from a sufficient number of satellites or in case of bad satellite geometry. Such methods have been described as hybrid positioning and generally involve an either/or selection between the outputs of two or more positioning technologies including GPS, A-GPS, WiFi and Cellular-based positioning, for example.